Biogas-fuelled aqua-ammonia diffusion absorption refrigeration: Thermodynamic analysis, empirical models, and predictions for anaerobic digestion reactor volume and feedstock regime

Abstract

• Innovated a biogas-fuelled aqua-ammonia DAR refrigerator prototype. • Proposed empirical model to predict biogas use in single-effect DAR refrigerators. • Achieved refrigerated space temperatures of 6.4 to -19 °C at varied biogas flows. • Demonstrated the suitability of biogas-fuelled aqua-ammonia DAR for rural off-grid usage. • A high feed biogas yield and solids content showed reduced digester size and feed flow. Post-harvest losses are a common challenge in developing countries, particularly in rural areas with limited cold chain capacities. To address this issue, aqua-ammonia diffusion absorption refrigeration, utilising biogas from anaerobic digestion, offers an off-grid cold storage solution with potential to enhance rural cold chain capability and reduce post-harvest losses. This study introduces a prototype as a proof-of-concept, assessing biogas consumption rates and thermodynamic performances in aqua-ammonia diffusion absorption refrigerators. The prototype achieved cold-space temperatures ranging from 6.4 to -19 °C at standard biogas flow rates of 0.56 to 0.81 Nm 3 biogas .m -3 refrigerated space .day -1 , representing a 250% v/v increase compared with LPG. Steady-state cooling capacities at various biogas flow rates were captured in a novel empirical model of biogas consumption, applicable to refrigerators with capacities of up to 250 L. The combination of experimental data and the developed model was used to predict the required reactor volume and feed flow rate for an anaerobic digestor to fuel a diffusion absorption refrigeration system. Both reactor volume and feed flow rate were minimised at the highest biogas yield (302 mL g VS -1 ) and total solid content (22.9%). Hence, integrating anaerobic digestion with aqua-ammonia diffusion absorption refrigeration provides a reliable short- to medium-term cold storage solution that can effectively reduce post-harvest losses in rural areas of developing countries.